Performance of Mechanochemically Activated Catalysts Is Enhanced by Suppression of the Thermal Effects of Ultrasound.

In this work, we demonstrate that the performance of mechanochemically activated transesterification and alkene metathesis catalysts is significantly enhanced when the thermal effects of ultrasound are suppressed. Suppression of these effects is realized by performing the reaction under methane instead of argon. Not only do these results provide further confirmation of the true mechanochemical nature of the ultrasonic activation of the catalysts, but it also strongly recommends the use of methane as standard saturation gas when studying the mechanochemical effects of ultrasound.

Mechanism of ultrasound scission of a silver-carbene coordination polymer.

Scission of a supramolecular polymer-metal complex can be carried out using collapsing cavitation bubbles created by ultrasound. Although the most plausible scission mechanism of the coordinative bonds is through mechanical force, the influence of radicals and high hot-spot temperatures on scission has to be considered. A silver(I)-N-heterocyclic carbene complex was exposed to 20 kHz ultrasound in argon, nitrogen, methane, and isobutane saturated toluene. Scission percentages were almost equal under argon, nitrogen, and methane. Radical production differs by a factor of 10 under these gases, indicating that radical production is not a significant contributor to the scission process. A model to describe the displacement of the bubble wall, strain rates, and temperature in the gas shows that critical strain rates for coil-to-stretch transition, needed for scission, are achieved at reactor temperatures of 298 K, an acoustic pressure of 1.2 × 10(5) Pa, and an acoustic frequency of 20 kHz. Lower scission percentages were measured under isobutane, which also shows lower strain rates in model simulations. The activation of the polymer-metal complexes in toluene under the influence of ultrasound occurs through mechanical force.

Probing single enzyme kinetics in real-time.

Studies of reaction kinetics at the single enzyme level have become possible due to the swift development of single molecule detection techniques. In this tutorial review, which targets both general and expert research chemists in the biochemical or physical (organic) chemistry field, we briefly introduce some of these fluorescence microscopic techniques and discuss selected studies on single molecule enzymology. The nature of the frequently observed fluctuations in the reaction rate, the so-called dynamic disorder, is described based on various available models.